By the time iSCSI became a standard protocol five years ago, most large enterprises had already adopted Fibre Channel...
(FC) as the network technology to connect their servers and storage devices. And now that users have spent an estimated $60 billion on FC to transport commands and data across their storage-area networks (SANs), they're not likely to rip out their battle-tested infrastructures.
But iSCSI SANs are finding favor among businesses of varying sizes that resisted shared storage because of Fibre Channel's cost and complexity. These iSCSI SAN solutions are especially popular for working with Windows-based files, database and application servers. They're also increasingly a consideration among large enterprises for departmental applications, server workloads that aren't bandwidth-intensive and for virtual servers.
The connection between shared storage and virtualized servers "has changed the game in the last year and a half," said Rick Villars, a storage analyst at IDC.
The pros and cons of iSCSI SAN solutions vs. Fibre Channel
Weighing the merits of iSCSI SAN solutions starts with the concept of shared storage. IT organizations that choose to separate storage from servers generally do so to better use and allocate resources and manage and protect data. SCSI is the standards-based interface, or command set, that connects the servers and storage arrays and transfers data across the network. The SAN provides a central pool of networked storage resources.
The physical storage is broken up into logically smaller chunks, and each chunk is assigned a SCSI logical unit number (LUN). The SCSI LUN appears to the application server as if it were a local disk drive, even though it's in reality across the network.
That leaves the major distinction between iSCSI and Fibre Channel in the transport and network technologies. An iSCSI SAN uses the Internet Protocol over Ethernet, whereas in the traditional FC SAN architecture, the Fibre Channel Protocol (FCP) handles the transport of SCSI commands over a Fibre Channel network.
Users find iSCSI easier to install and manage than Fibre Channel, not to mention much cheaper to deploy. Software- or hardware-based initiators send the SCSI commands over the IP network. Software initiators ship with all major operating systems, reducing the cost associated with setting up an iSCSI SAN. Performance-boosting hardware initiators, implemented through iSCSI host bus adapters (HBAs), are also available, but they can add $200 to $500 to the price of a low-end SAN.
"The value proposition of iSCSI is around low cost," said Greg Schulz, founder of storage consulting firm StorageIO Group. "That's why you see the vast majority of installs with software-based initiators that are built right in." Schulz estimates that 85% to 90% of installs are done with software initiators.
That's one of the tipping points between Fibre Channel and iSCSI. Prices for FC's host bus adapters, which run the network protocol and SCSI protocol, start at approximately $500 but go higher, depending on performance and the number of ports. Per-port switches, cabling and server-side connectivity are also often more expensive.
Complicating matters further, IT managers need special knowledge to run a Fibre Channel network. With an iSCSI SAN solution, they can simply leverage the experience and investments they've collected with their existing Ethernet and IP networks, even though they still need to learn the nuances of storage allocation.
Fibre Channel, on the other hand, was designed to handle transaction-heavy loads reliably with no data loss and low latency. Most companies currently use 4 Gbps Fibre Channel, although 8 Gbps is starting to gain ground. Either way, there's more bandwidth with Fibre Channel than with the 1 Gigabit Ethernet that most iSCSI SANs now employ.
Fans of iSCSI can argue that bandwidth isn't the sole determiner, and they can get ample performance with many workloads. Other factors in the performance equation include the number of processors, host ports, cache memory, disk spindles or drives, and how wide they can be striped.
On the storage end, users don't need to make an either/or choice. They can opt for multiprotocol arrays, or unified storage, that support both iSCSI and FCP for block-based storage and data access with SANs and NFS and CIFS for file-based storage and data access with network-attached storage (NAS).
"Why limit yourself to a single protocol when you're not sure that protocol is going to last forever?" asks George Crump, founder of consulting firm Storage Switzerland. Crump says he's hesitant to recommend an iSCSI-only target, or storage pool, to clients.
Simultaneous deployment is the wave of the future, according to Robert Passmore, an analyst at Stamford, Conn.-based Gartner Inc. He's already witnessing large companies eliminate file servers and consolidate file services to NetApp Inc.'s NAS device, put low-end servers that don't need performance onto iSCSI and leave the mission-critical, highest-performing applications on Fibre Channel.
iSCSI solutions and virtual servers
A hybrid approach also is helpful to enterprises that add virtual servers into the mix. Server virtualization technology frees a company to host many instances of an operating system on a single physical box, and administrators can shift server workloads from one box to another to optimize resources. However, that becomes difficult if the storage sits on the same box.
In virtual environments, iSCSI SAN solutions can claim an inherent advantage over Fibre Channel SANs with virtual machine (VM) mobility when combined with the ability to provision virtual servers from SAN-bootable devices, according to Jeff O'Neal, senior director of data center solutions at NetApp. Also, backups to tape or disk can be done directly from a guest operating system, or a VM, with an iSCSI SAN, whereas a Fibre Channel SAN adds a layer of management to the virtual machine hypervisor, he noted.
"The VMware adoption curve has been so steep that it's driven more and more companies to use a SAN where they might not have done so before," said Andrew Reichman, an analyst at Forrester Research Inc. Other options include configuring VMware to use NFS with NAS devices or choosing a NAS gateway in front of a traditional block-based storage array.
Reichman recommends that companies with no Fibre Channel look at iSCSI solutions, and even companies with Fibre Channel consider iSCSI to network additional servers at a lower cost. "Going iSCSI now will make it a smoother transition to 10 Gigabit Ethernet [10 GbE] later," he said.
But iSCSI SAN users won't concern themselves with that performance boost until the price of 10 Gigabit Ethernet drops over the next few years. "The reason iSCSI is doing well in the market is because of cost savings, not because it could or couldn't do the job that Fibre Channel did," Gartner's Passmore said. "If you look at 10 Gigabit Ethernet today, the cost of the components is still well above Fibre Channel costs. You get performance that's not needed in the market that's buying iSCSI."
So far, 10 Gigabit Ethernet is finding a captive audience only in select iSCSI scenarios, such as between the storage array and the switch. The array's ability to use the higher bandwidth can reduce the number of ports, offsetting the higher cost of 10 Gigabit Ethernet, Passmore said. On the initiator side, because few servers need more than 1 Gbps for storage traffic, users won't opt for 10 GigE there until it becomes economical.
But even cheaper 10 Gigabit Ethernet won't enable iSCSI to eliminate Fibre Channel. Building large, complex iSCSI SANs can be tough because of the dearth of management tools and industrial-strength test configurations from vendors. For large enterprises, Fibre Channel over Ethernet (FCoE) technology might eventually be a more palatable option.
The Ethernet, however, won't be the garden variety in use today. Typical Ethernet needs high-overhead TCP/IP to provide for retries and acknowledgements and a level of flow control in delivering information. Even then, it's not "lossless," with a guarantee that no frames or packets will be dropped.
Cisco Systems Inc., Brocade Communications Systems Inc. and other industry players are working on standards to enhance Ethernet for data center use, adding lossless characteristics and high-performance transport of multiple network traffic types. Converged Enhanced Ethernet (CEE) standards are expected next year, and the technology will need a year or two more to mature, according to Mario Blandini, Brocade's director of product marketing for data center infrastructure.
Once products supporting CEE hit the market and prices drop for 10 Gigabit Ethernet, all storage traffic could shift to Ethernet. Fibre Channel users would get to use the same management tools and leverage their existing skills and experience, much the same as iSCSI users do now at the low end.
"Fibre Channel, at least for the foreseeable future, is the de facto standard for a mid- to upper-size SAN," Passmore said. "We estimate users spent $60 billion getting where they are today. [They're not] going to unplug it next week just to be able to spend slightly less money. But that doesn't mean those same companies might not want to connect to other servers in the SAN using iSCSI -- and that's just what's happening."
About the author: Carol Sliwa is a veteran IT journalist.